The invention relates to radiation curable polymers and formulations or compositions containing them, as well as photosensitive articles having solid surfaces or layers prepared from such polymers or formulations. The invention also relates to a process for making and using the polymers and making and using the photosensitive articles.
Photocurable polymers and compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. Generally, photocured printing plate comprise a support and a photosensitive surface or layer of photocurable composition. To prepare the printing plate the photosensitive surface is exposed to radiation in an imagewise fashion. The unexposed areas of the surface are then removed in developer baths.
In the past, removal of the unexposed surfaces has required the use of organic solvents which are environmentally unsafe, toxic and highly volatile. Thus, there has been a need to develop photocurable compositions which can be developed in non-organic solvent. In addition to possessing an aqueous photodevelopable photosensitive surface, a flexographic printing plate must be sufficiently flexible to wrap around a printing cylinder, while being strong enough to withstand the rigors of the typical printing process. Furthermore, the printing plate must be soft enough to facilitate ink transfer during printing and must be solvent resistant to inks typically used in printing. U.S. Pat. No. 5,328,805, Huynh-Tran, et al., xe2x80x9cAqueous Developable Photosensitive Polyurethane(METH)-acrylatexe2x80x9d (1994) and U.S. Patent No. Huynh-Tran, et al., xe2x80x9cAqueous Developable Photosensitive Polyurethane(METH)-acrylatexe2x80x9d (1994) teach photopolymer resins comprised of a urethane prepolymer prepared by reacting polyoxyalkylene diols with polyester diols, or a mixture thereof, with an excess of diisocyanate followed by chain extending the resulting prepolymer mixture with an alkyldialkanolamine, then reacting the chain-extended product with a hydroxyalkyl(meth)acrylate.
However, improved mechanical properties such as resistance to inks including oil, water and alcohol based inks is still desired. Incorporation of butadiene would yield more flexible, softer compositions. However, until now, urethane prepolymers have not incorporated butadiene because butadiene polymers are generally incompatible with typical urethanes resulting in a cloudy product which causes light scattering during the imagewise exposure step. Light scattering results in poor image quality in the resulting printing plate.
The printing plates of the present invention overcome this limitation, incorporating butadiene into urethane prepolymers and resulting in photopolymer products which are more resistant to inks, have improved resilience, improved cold-flow properties and lower surface tension.
A photopolymer resin composition comprising a polyether-polybutadiene prepolymer is provided by the present invention.
Methods of preparing photopolymer resins of the present invention are also provided comprising reacting to completion hydroxy-terminated butadiene homopolymer with isocyanate to produce isocyanate-terminated polybutadiene polyurethane; reacting isocyanate-terminated polybutadienepolyurethane with polyether diol to produce prepolymer; reacting prepolymer with 0 to 25 parts by weight of a hydroxyalkyl(meth)acrylate; reacting the products with 1-20 parts by weight of alkyldialkanolamine in the presence of solvent; and adding a photoinitiator, stablizers and UV absorber. Printing plates of the present invention may be prepared by casting photopolymers of present invention onto a plate substrate and drying the plate to remove the solvent.
It is an object of the invention to provide novel solid, water-dispersable polymer which can be cross-linked or cured by exposure to actinic radiation.
It is a further object of the invention to provide aqueous-developable flexographic relief printing plates and methods of making the same.
Another object of the invention is to provide a novel butadiene urethane prepolymer.
Photopolymer resins of the present invention comprise polyether-polybutadiene prepolymer and photoinitiator.
As general guidance in preparing compositions of the present invention, the milliequivalents of isocyanate added should be equal to or greater than the total milliequivalents of the remaining components, namely polybutadiene, polyether diol, alkyldialkanolamine, and hydroxyalkyl(meth)acrylate. However, it is generally preferred that the ratio of milliequivalents of isocyanate to non-isocyanate components be approximately equal. Alkyldialkanolamine generally is about 15-60% of the non-isocyanate components (based upon total milliequivalents) and preferably about 55-60%. Hydroxyalkyl(meth)acrylate makes up about 7-40% of the total non-isocyanate components, and preferably is about 8%.
To prepare compositions of the present invention hydroxy-terminated polybutadiene is reacted at about 25-80xc2x0 C. with a stoichiometric excess of diisocyanate in the presence of a urethane forming catalyst to produce isocyanate terminated polybutadiene polyurethane. Thereafter, the isocyanate terminated polybutadiene polyurethane is mixed with polyether diol to form a prepolymer. The ratio of polybutadiene to polyether diol is from 1:1 to 1:5, and preferably 1:3. The prepolymer is reacted with hydroxyalkyl(meth)acrylate at a temperature of from about 60-80xc2x0 C. Further reaction with an alkyldialkanolamine is performed in the presence of an organic solvent. Suitable solvents include methyl methyl ketone, methyl isobutyl ketone, toluene, and mixtures thereof. The solvents should have a boiling point in the range of 80-120xc2x0 C. for easy casting of the films and evaporation of the solvent. Thereafter a curing agent is added.
Hydroxy-terminated Polybutadiene
Hydroxy-terminated polybutadiene has the formula 
where x+y+z is 1 and n is from about 25 to about 50. It is preferred that x is 20%, y is 20% and z is 60% of the overall molecule. In preferred embodiments n is 50. Suitable polymers have a number average molecular weight in the range of about 1,000 to 10,000, preferably in the range of about 1,000 to 3,000. As used herein molecular weights are numbered average, Mn, as determined by Permeation Chromatography, using polystyrene standards. Hydroxylated polybutadiene is available from commercial sources such as Elf Atochem.
Polyether Diol
Polyether diols have the formula OHxe2x80x94(Axe2x80x94Oxe2x80x94)bH where A is a divalent radical of ethylene, propylene, isopropylene, butylene, isobutylyene; and b is such that the numbered average molecular weight of the group [Axe2x80x94O]b or of the polymer diol, is within the range of about 650 to about 7,000, preferably about 1,000-3,500. Ethylene oxide block polyether diol where the amount of ethylene oxide is 1-50 weight percent, preferably 10-30 weight percent and the overall molecular weight, 1,000 to about 7,000, preferably 1,000 to 4,000 are particularly preferred. In the instance where copolymers are used, the structure for A(O)b would be xe2x80x94(CH2CH20)xxe2x80x94(CH2CH(CH3)xe2x80x94O)yxe2x80x94(CH2CH20)zxe2x80x94 where x+z and y are such that the ethylene oxide moiety represent about 1-50, preferably 10-30 weight percent of the overall polyol molecular weight. Such copolymers include POLY G55-37 (MW 2952, containing about 30 weight percent of ethylene oxide), and POLY L 228-28(MW 4000, containing about 20 weight percent ethylene oxide), both available from Olin Corporation. The polyether diol reactants may be made by processes well known in the art by reacting an alkylene oxide or mixtures of alkylene oxides with a compound having at least one active hydrogen atom such as water, monohydroxylic alcohols such as ethanol and propanol; and dihydroxylic alcohols such as ethylene glycol and monoethyl ether of glycerine. The poly(oxyalkylene) products of such reactions will have linear oxyalkylene or oxyethylene higher oxyalkylene chains, and such chains will terminate with hydroxyl groups. Conventional potassium hydroxide catalysts or double metal cyanide catalysts can be used.
Diisocyanate
A wide variety of diisocyanates is available. These diisocyanates can be aliphatic, cycloaliphatic or aromatic, with the structure OCNxe2x80x94R1NCO. The divalent radical R1 contains in general 2 to 100 carbon atoms and may optionally carry non-interfering substituents such as ether groups, ester groups, urethane groups, amido groups, urea groups, aryl groups, aliphatic groups, cycloaliphatic groups, or halogen atoms. Modified diisocyanates are also usable. Examples of suitable diisocyanates include 4,4xe2x80x2-methylene diphenyl diisocyanate (MDI) and MDI prepolymers, 2,4-tolylene diisocyanate (toluene diisocyanate), 2,6-tolylene diisocyanate, mixtures of the two, dimers of 2,4-tolylene diisocyanate, p-xylene diisocyanate, m-xylene diisocyanate, 1,5-naphthalene diisocyanate, 3, 3xe2x80x2-dimethylbiphenyl-4, 4xe2x80x2-diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, lysine diisocyanate, lysine diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-methylenebis(cyclohexylisocyanate), methyl-cyclohexane-2,4 (or 2,6)-diisocyanate, 1,3-isocyanatomethyl) cyclohexane, m-tetramethylxylene diisocyanate, p-tetramethylxylene diisocyanate, and mixtures thereof.
Catalyst
Conventional urethane forming catalysts may be used in the reactions with diisocyanates. These catalysts include, for example, organo-tin compounds such as dibutyl tin dilaurate and stannous octoate, organomercury compounds, tertiary amines, and mixtures of these materials.
Alkyldialkanolamine
Alkyldialkanolamines of the present invention have the structure HOR2R3R4OH, where the numbered R2 and R4 are C1-C6 alkylene or R2 is xe2x80x94(CH2CH2O)dCH2CH2xe2x80x94 provided R4 is xe2x80x94(CH2CH2O)eCH2CH2xe2x80x94 where d and e are 0-9 and d+e is 3-15. R3 is xe2x80x94N(R7)xe2x80x94, xe2x80x94NNxe2x80x94, or xe2x80x94N(Ph)xe2x80x94 and R5 is C1 to C6 alkyl group.
Within this group, methyldiethanolamine bis(hydroxylethyl)-piperazine, 
and N,Nxe2x80x2-bis(2-hydroxypropyl)aniline, HOxe2x80x94CHxe2x80x94(CH3)CH2xe2x80x94N(Ph)xe2x80x94CH2CH(CH3)OH are preferred. (Ph=phenyl.)
Hydroxyalkyl(meth)acrylate
This material has the structure HOxe2x80x94R6OC(xe2x80x94:O)xe2x80x94C(R7)xe2x95x90CH2, where R6 is C1-C7 alkylene and R7 is H or methyl. Within this structure polypropylene glycol monomethacrylate is preferred. Unless otherwise indicated, the term xe2x80x9c(meth)acrylatexe2x80x9d means either acrylate or methacrylate.
Formulations with the Invention Photopolymer
The simplest formulation is the invention photopolymer plus an effective amount of photo initiator. Such mixture can be solvent cast, as is, or the solvent removed and the mixture extruded to create a solid photopolymerizable layer on conventional backing material.
Diluent
For many commercial purposes it will be found preferable to formulate or extend the photopolymerizable prepolymer composition with about 1% to about 30% by weight of the prepolymer of a reactive (i.e., photoactive) monomer or oligomer, and most preferably in the range of 5 to about 15% by weight reactive monomer. Suitable reactive monomers or oligomers are mono, di or multiacrylate diluents of the formula:
(CH2xe2x95x90C(R7)xe2x80x94C(O)xe2x80x94Oxe2x80x94)qR8
where R7 is H or methyl, R8 is an organic moiety having a valence of q, and q is an integer.
Such reactive (meth)acrylate diluents include, but are not limited to, trimethylolpropane triacrylate, hexanediol diacrylate, 1,3-butylene glycol diacrylate, diethylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, polyethylene glycol 200 diacrylate, tetraethylene glycol diacrylate triethylene glycol diacrylate, pentaerythritol, tetraacrylate, tripropylene glycol diacrylate, ethoxylated bisphenol-A diacrylate, propylene glycol mono/dimethacrylate, trimethylolpropane diacrylate, di-trimethylolpropane tetracrylate, triacrylate of tris(hydroxyethyl) isocyanurate, dipentacrythritol hydroxypentaacrylate, pentaerythritol triacrylate ethoxylated trimethylolpropane triacrylate, triethylene glycol dimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, polyethylene glycol-200 dimetharcrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycol-600 dimethacrylate, 1,3-butylene glycol dimethacrylate, ethoxylated bisphenol-A dimethacrylate, trimethylolpropane trimethacrylate, diethylene glycol dimethacrylate, 1,4-butanediol diacrylate, diethylene glycol dimethacrylate, pentaerythritol tetramethacrylate, glycerin dimethacrylate, trimethylolpropane dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol diacrylate, urethanemethacrylate or acrylate oligomers and the like which can be added to the photopolymerizable composition to modify the cured product. Monoacrylates such as cyclohexyl acrylate, isobornyl acrylate, lauryl acrylate and tetrahydrofurfuryl acrylate and the corresponding methacrylates are also operable as reactive diluents, as well as methacrylate oligomers such as epoxy acrylates, urethane acrylates, and polyester or polyether acrylates.
Photoinitiators
The formulations comprising the novel materials of this invention require a photoinitiator. A large number are available and useful.
Photoinitiators for the photopolymerizable composition and formulations containing the same include the benzoin alkyl ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether and benzoin isobutyl ether. Another class of photoiniators are the dialkoxyacetophenones exemplified by 2,2-dimethoxy-2-phenylacetophenone, i.e., Irgacure(copyright)651 (Ciba-Geigy) and 2,2-diethoxy-2-phenylacetophenone. Still another class of photoiniators are the aldehyde and ketone carbonyl compounds having at least one aromatic nucleus attached directly to the carboxyl group. These photoiniators include, but are not limited to, benzophenone, acetophenone, o-methoxybenzophenone, acetonaphthalene-quinone, methyl ethyl ketone, valerophenone, hexanophenone, alpha-phenyl-butyrophenone, p-morpholimopropiophenone, dibenzosuberone, 4-morpholinobenzophenone, 4xe2x80x2-morpholinodeoxybenzoin, p-diacetylbenzene, 4-aminobenzophenone, 4xe2x80x2,-methoxyacetophenone, benzaldehyde, alpha-tetralone, 9-acetylphenanthrene, 2-acetylphenanthrene, 10-thioxanthenone, 3-acetylphenanthrene, 3-acetylindone, 9-fluorenone, 1-indanone, 1,3,5-tricetylbenzene, thioxanthen-9-one, xanthene-9-one, 7-H-benz[de]-anthracen-7-one, 1-napthaldehyde, 4,4xe2x80x2-bis(dimethylamino)-benzophenone, fluorene-9-one, 1xe2x80x2-acetonaphthone, 2xe2x80x2-acetonaphthone, 2,3-butedione, acetonaphthene, benz[a]anthracene 7.12 diene, etc. Phosphines such as triphenylphosphine and tri-o-tolyl-phosphine are also operable herein as photoinitiators. The photoinitiators or mixtures thereof are usually added in an amount ranging from 0.01 to 10% by weight of the total composition.
Other Additives
The compositions may also contain other additives which are known in the art for use in photocurable compositions, e.g., antioxidants, antiozonants and UV absorbers. To inhibit premature crosslinking during storage of the prepolymer containing compositions of this invention, thermal polymerization inhibitors and stabilizers are added. Such stabilizers are well known in the art and include, but are not limited to, hydroquinone monobenzyl ether, methyl hydroquinone, amylquinone, amyloxyhydroquinone, n-butylphenol, phenol, hydroquinone monopropyl ether, phenothiazine, phosphites, nitrobenzene and phenolic-thio compounds, and mixtures thereof. Such additives are used in an amount within the range of from about 0.01 to about 4% by weight of the prepolymer. These stabilizers are effective in preventing crosslinking of the prepolymer composition during preparation, processing and storage.
UV light absorbers, or UV light stabilizers, can be used to adjust the photospeed and, therefore, exposure latitude of the polymer material. Numerous materials will be apparent to those skilled in the art.
The most important light stabilizer classes are 2-hydroxy-benzophenones, 2-hydroxyphenyl benzotriazoles, hindered amines and organic nickel compounds. In addition, salicylates, cinnamate derivatives, resorcinol monobenzoates, oxanilides, and p-hydroxy benzoates are used. These additives are used in the range of about 0.09 to about 4% by weight of the prepolymer.
Tinuvin(copyright) 1130, a substituted hydroxyphenyl benzotriazole, available from Ciba-Geigy Corp., has been found to work exceptionally well.
The compositions also may contain up to about 50% by weight of an inert particulate filler which is essentially transparent to actinic light. Such fillers include the organophilic silicas, bentonites, silica and powdered glass. Such fillers can impart desirable properties to the photocurable compositions and reliefs on printing plates containing those compositions.
The compositions may also contain dye and/or pigment coloring agents. The colorants present in the photopolymer composition must not interfere with the imagewise exposure and should not absorb actinic radiation in the region of the spectrum that the initiator, present in the composition is activatable.
The colorant may be chosen from among the numerous commercially available pigments and dyes. The coloring agent may be used in a solvent soluble form, or in the form of a dispersion. Where a particulate material is used, the particle size should be less than 5000 Angstroms. More preferably, the particles will be in the 200-3000 Angstrom range.
Although numerous pigments and dyes useful in the practice of the present invention will be apparent to those skilled in the art, a small number of such materials are listed here.
Suitable pigments include the Microlith(copyright) series available from Ciba-Geigy. Especially preferred are the A3R-K and 4G-K materials. Suitable dyes include for example, Baso Blue 645 (C.I.Solvent Blue 4), Baso Blue 688 (C.I. Solvent Blue 81), Luxol Fast Blue MBSN (C.I. Solvent Blue 38), Neopen Blue 808 (C.I. Solvent Blue 70), Orasol(trademark) Blue 2GLN (C.I. solvent Blue 48), Savinyl(copyright) Blue GLS (C.I. Solvent Blue 44), Savinyl Blue RLS (C.I. Solvent Blue 45), Thermoplast Blue 684 (C.I. Solvent Violet 13) and Victoria Blue BO (C.I. Solvent Blue 7). Blue and violet are used in current applications but the color of the dye is not critical. Other colors could offer advantages, including for example, resistance to fading.
Formulations using the photopolymers of this invention include the following (in parts by weight):
(1) Photopolymer, about 50-100, preferably about 70-90;
(2) A mono-, di-, or multi(meth)acrylate diluent, which can be a monomer or oligomer, about 0-25, preferably about 5-15;
(3) Photoinitiator, about 0.1-10, preferably about 0.5-2.0;
(4) Organic solvent, 0 to about 200, preferably about 10-50; and
(5) Stabilizers, UV absorbers, and colorants, 0.1-10, preferably about 1-4, total.
Preparation of Plate
The photocurable composition can then be shaped and formed as a solid layer of suitable thickness according to conventional solvent casting, i.e. dissolving the composition in a solvent, shaping the solution into a film or plate and removing the solvent. Conventional extrusion, calendaring or hot press techniques can also be used. Solid layers of the photosensitive composition in the form of a film can be adhered to supports such as those comprising polyester, nylon or polycarbonate. Other suitable supports include woven fabrics and mats, e.g. glass fiber fabrics or laminated materials made of, for example, glass fibers and plastics, and steel or aluminum coated plates. It is preferred that the supports are dimensionally stable and resistant to the washout solution.
It is also usually necessary to protect photosensitive surfaces from contamination by dirt and dust during storage before being exposed and washed. Such protection is accomplished by lamination or application of a flexible protective cover sheet to the side of the photocurable composition opposite that of the support. In addition, the photocurable compositions can sometimes be tacky and it is thus also desirable to apply a release film to the surface of the photosensitive layer before application of the cover sheet. The release film consists of a thin, flexible and water soluble polymeric film and allows for intimate contact between the surface of the photocurable composition opposite the support and an image-bearing negative applied to the surface.
Exposure and Development
Photosensitive articles comprising a support having a solid layer or surface comprising the photocurable composition, e.g., solid flexographic printing plates, can then be processed by well-known techniques for imagewise exposure to actinic light. Preferably, the light should have a wavelength of from about 230-450 microns. Exposure is through a negative placed between the light source and the photosensitive surface. Suitable sources of light include Type RS sunlamps, carbon arc lamps, xenon arc lamps, mercury vapor lamps, tungsten halide lamps and the like.
Exposure periods depend upon the intensity of the actinic light, thickness of the plate and the depth of the relief desired on the printing plate. Periods of from 1 to 20 minutes exposure are preferred.
After exposure and removal of the negative, the unexposed areas of the photosensitive surface can be developed (removed) in aqueous washout solutions as herein described. This feature is particularly advantageous as it avoids problems of disposing of washout solutions containing commonly used organic solvents, such as chlorinated solvents, alcohols or ketones. The washout solution should be slightly acidic and may contain a surfactant. Dilute vinegar or lactic acid solutions are preferred. Useful acidic surfactants include sodium alkynaphthalene-sulfonate, sodium alkylbenzene sulfonate, sodium alkylether sulfate, polyoxyalkylated alkylaryl phosphate ester sodium salt and the like. Overall additive concentrations are suitably 0.1-5%. Wash temperature can vary from 25-790xc2x0 C., preferably at ambient temperature. Following washout, the plate may be postexposed for further hardening of the relief work.